The present invention relates to an electromagnetic actuator having a permanent magnet.
Document U.S. Pat. No. 4,295,111 discloses an electromagnetic actuator comprising a coil placed around a magnetic body in which a magnetic actuator member is mounted. A permanent magnet is placed around the coil and has a magnetic field extending along an axial direction of the coil.
The permanent magnet is interposed between two rings of magnetic material which are in contact with the magnetic body. This serves to concentrate the magnetic field and to develop a high level of force. In contrast, the stroke of the actuator member is relatively short, and for long strokes, it is necessary to increase the axial dimensions of the actuator.
The invention provides an electromagnetic actuator comprising a coil, a magnetic actuator member mounted in the coil, at least one ring of magnetic material placed around the coil, and at least one permanent magnet adjacent to one end of the ring and disposed overlying a portion of the outside surface of the coil, the permanent magnet having a magnetic field that extends in an axial direction of the coil, the magnetic actuator member being slidably mounted in a non-magnetic guide member extending inside the coil, and the ring having an end opposite from the magnet which is spaced apart from the non-magnetic guide member by a non-magnetic gap.
Thus, instead of being totally looped, a portion of the magnetic field remains open so that the stroke of the magnetic actuator member is lengthened. It is then possible to control the position of the magnetic actuator member by appropriately controlling the power supply to the coil.